The present invention is an improvement in portable X-ray tomography systems such as those described in U.S. Pat. Nos. 4,928,283 (issued May 22, 1990 to Bernard M. Gordon for X-Ray Tomography Apparatus, and assigned to the present assignee) and 5,109,397 (issued Apr. 28, 1992 to Bernard M. Gordon et al. for X-Ray Tomography Apparatus With Lateral Movement Compensation, also assigned to the present assignee); and helical scan tomography systems such as described in German Gebrauchsmuster File No. G 87 03 190.6, filed by Siemens AG and published Aug. 11, 1988, all three documents being incorporated herein by reference.
Tomography systems have been used for many years to create images of cross-sectional slices through objects, and are particularly useful as a diagnostic aid. CT (computed tomography) scan systems usually include a gantry assembly comprising a disk mounted for rotation within a gantry frame. In third generation machines, the disk supports X-ray imaging components including an X-ray source and X-ray detectors that rotate within a stationary frame. In fourth generation machines the X-ray detectors are secured equiangularly around the stationary frame, while the source rotates with the disk relative to the detectors. In both types of systems, the source may provide periodic X-ray pulses, or alternatively, continuous-wave (CW) X-rays. The disk of the gantry is normally adapted to rotate through a full 360.degree. rotation so that the imaging components secured to the disk rotate through a plurality of incremental positions where a corresponding series or set of readings (called "views") by the detectors are made. The number of photons absorbed along the various paths through the object, during each sampling period defining each view or set of readings, is a function of the absorption characteristics of the portions of the object along each path during each set of readings. Thus, a plurality of views are taken through the portion of an object disposed within the common plane of rotation of the X-ray paths (hereinafter the "scanning plane"). The detectors generate a corresponding plurality of analog information signals representative of X-ray flux detected by the detectors during each sampling period or projection view.
The output analog information signals of the X-ray detectors acquired from all of the views of the 360.degree. rotation, i.e., through all of the incremental angular positions of the 360.degree. rotation within the scanning plane, are processed, typically through a back projection processing technique, so as to create an image of the slice of the interior structure of the object exposed to the X-rays. In some CT scanners, provision is made to move the patient support while performing a scan in order to provide a helical scan so as to increase the scanned volume.
An important consideration in CT scanning has always been the accurate and consistent alignment of the tomography components and the patient both throughout the rotation and over the course of many scans and patients. Misalignment or movement can negatively influence the data of an entire scan. In order to deal with this factor, manufacturers of prior art CT scan apparatus typically have produced a very large and massive machine which includes a heavy gantry assembly for supporting the tomography components. The handling of this weight requires additional mass in the remainder of the apparatus and typically a large apparatus overall. The rotating disk supporting at least the X-ray source is typically rotatably supported in a massive, finely-machined bearing assembly, and an extremely massive and heavy support system is provided for supporting the gantry assembly and tomography components. The very massiveness of these systems helps to minimize vibration and other lateral mechanical movements, which, for example, can occur with wear between parts moving relative to one another.
But such massive systems are also extremely expensive to build and once located and constructed for use are extremely difficult to relocate. They require large amounts of floor space and thus can not be used in space limited environments. Thus, use of such systems, for example, within the operating theater are impractical. The result is that a very powerful data gathering diagnostic system is not readily available to a surgeon when such data might be very helpful, or to trauma units when movement of the patient should be minimized.
One ramification of the extra size and mass of these machines has been to provide a fixed gantry assembly so as to require a movable patient table, as the weight of the patient and the patient table is typically much less than the weight of the gantry assembly and its rotating components. A movable patient table is used not only to properly position the patient in the desired location relative to the fixed gantry assembly so that a scan can be performed through a select portion of the patient's body; but in the case of machines capable of performing helical scans, to move the patient parallel to the rotation axis (referred to as the "Z-axis") of the tomography imaging components, while the components are rotating about the patient. But, the apparatus which has resulted from these various requirements has been typically large, heavy, expensive, and difficult to relocate. It has required a large amount of floor space and thus can not be used in space-limited environments. A further disadvantage caused by the size and weight of these apparatuses has been the wear experienced in the moving parts thereof. Moreover, in the course of repositioning a movable patient table for successive scans or a helical scan, the momentum of the movement of the table can result in patient movement during the scan resulting in erroneous scanning data. This problem can be especially acute when the table moves abruptly at the beginning or end of a scan.
At least one three dimensional CT scanning system has been suggested in German Gebrauchsmuster File No. G 87 03 190.6 (described above) in which the tomographic elements move through a helical path about a stationary patient table. As described in the document, a rotating ring supports the X-ray source and X-ray receiver. The rotating ring is provided with helical screw threads so that as the ring rotates within a shell during a scan it simultaneously moves parallel to the patient table so that the ring moves in a helical manner along the length of the patient table and so that a high three-dimensional volume can be measured. The data acquired during a three dimensional scan make it possible to reconstruct large volume images. However, the movement of the rotating ring is confined to helical movement so that the system is only capable of three-dimensional scanning. Additionally, while the patent suggests that, depending on system design, it is possible to reconstruct images taken during the three-dimensional scanning process for any theoretical rotating ring inclination (layer angle) on the basis of data taken during the scan even through the scanning plane established by the ring can not be tilted. This would require the ring to be laterally displaced parallel to the orientation of the patient table over a large distance. For example, to obtain data equivalent to a 45.degree. inclined angle, the rotating ring would have to transverse a distance equal to the distance between the source and detectors exposing the patient to excessive amounts of X-rays over a large period of time. Further, the ring appears to be large, and unnecessarily massive. The system therefore would not be practical as a portable system.
The portable tomography apparatus described in U.S. Pat. No. 4,928,283 (Gordon) represents a significant improvement in the state of the art by providing an apparatus with a high degree of mobility. The tomography components are supported on a structure that also serves to support a removable patient table. The gantry assembly is movable between a first position where the table is disposed within the gantry and a scan can be performed on a patient disposed on the patient table and a second position where the gantry assembly is oriented 90.degree. to the first position so that it can be more easily transported and stored. The gantry assembly can be tilted. The support structure in the Gordon patent also provides means for rotating the tomography components about the Z-axis and means for moving the tomography components linearly along the Z-axis relative to the table when the tomography components are oriented in the first direction. The patent suggests that the apparatus is thus capable of helical scanning.
The apparatus of the Gordon patent is therefore an improvement over the much larger, essentially immobile tomography systems of the prior art. With the Gordon apparatus, the tomography system can be brought to the patient, for example, instead of vice versa. But, the apparatus of the Gordon patent is still larger and less maneuverable than desired because this apparatus necessarily incorporates the support structure for the patient table. Furthermore, this apparatus requires transferring a patient to the special removable patient table, which inhibits use of the apparatus at critical times, such as during surgery, or during emergency situations. In addition, precise control and measurement of the position of the gantry assembly relative to the patient is an elusive problem, since reducing the overall weight and mass of the system increases the amount of vibration and mechanical noise to which the system will be subject as the components rotate during a scan.
U.S. Pat. No. 5,109,397 (Gordon et al.) attempts to overcome some of the problems endemic to a portable tomography system by incorporating means for providing error information at incremental angular positions and means for compensating the data derived from the analog information signals. Another type of measurement system for providing such error information is described in U.S. patent application Ser. No. 08/162,653 filed on Dec. 6, 1993 in the names of Bernard M. Gordon, et al., for "Apparatus for and Method of Measuring Geometric, Positional and Kinematic Parameters of a Rotating Device" (Attorney's Docket No. ANA-21) and assigned to the present assignee. It remains desirable, nevertheless, to reduce or eliminate misalignment errors in portable tomography systems through more accurate control and measurement of the position and orientation of the tomography components.
It is desirable to provide an improvement over the X-ray system described in U.S. Pat. No. 4,928,283 (Gordon), by providing an improved X-ray tomography system with translation and pivoting control so that precise positioning of the components relative to a stationary patient table can be easily accomplished.